THIS APPLICATION IS A U.S. NATIONAL PHASE APPLICATION OF PCT INTERNATIONAL APPLICATION PCT/JP00/00548.
The present invention relates to a power supply circuit employed in a various kind of electronic appliances, telecommunications equipment, and the like, for generating a voltage pulse by resonance effect of a primary side of transformer, and outputting the voltage pulse from a secondary side after raising a potential thereof.
With reference to accompanying figures, a power supply circuit of the prior art will be described hereafter.
FIG. 16 represents a circuit diagram illustrating a power supply circuit of the prior art, and FIG. 17 is a drawing of waveforms showing changes in voltage, current, and switch pulse in the power supply circuit with time.
The power supply circuit of the prior art shown in FIG. 16 is intended to stabilize an output voltage of high potential applied by a transformer to a display. Its composition has been such that it comprises a driving power supply 403 connected to one side of terminals of a primary coil 402 of the transformer 401, and a switching element 404, a capacitor 405 and a diode 406 connected to the other side of the terminals of the primary coil 402.
The switching element 404 is comprised of a MOS type field-effect transistor (MOS FET) that has an internal diode. This MOS type field-effect transistor is disposed in a manner that a drain is connected to the other side terminal of the primary coil 402, a source is connected to a ground side, and a gate is connected to a PWM control circuit 407, which generates a pulse wave for controlling the switching element 404. It contains the internal diode with its anode connected to the ground side, and a cathode to the other side terminal of the primary coil 402. The capacitor 405 has its one end connected to the other side terminal of the primary coil 402, and the other end connected to the ground side. The diode 406 has its cathode connected to the other side terminal of the primary coil 402, and anode connected to the ground side. In addition, the cathode of the diode 406 and the one end of the capacitor 405 are connected to a point where the drain of the switching element 404 and the primary coil 402 make connection.
A display 409 (CRT) having high horizontal and vertical scanning frequency is connected to a secondary coil 408 of the transformer 401.
Furthermore, waveforms of voltage, current and switch pulse in this power supply circuit, as they change with a lapse of time are shown in FIG. 17.
In FIG. 17, a reference letter (a) represents a waveform illustrating a time series of change in value of voltage induced in the primary coil 402 of the transformer 401 taken at a point xe2x80x9cOxe2x80x9d in the power supply circuit; a letter (b) a waveform illustrating a time series of change in amount of current flowing at the point xe2x80x9cOxe2x80x9d in the power supply circuit; and a letter (c) a waveform illustrating a time series of change in shape of an output wave of the PWM control circuit fed to the switching element 404.
During a period of A to B in FIG. 17, when a pulse wave (the output wave) of a predetermined duration shown by the waveform (c) is input from the PWM control circuit 407 to the switching element 404, making the switching element 404 into an ON state, amount of electric current in the point xe2x80x9cOxe2x80x9d increases with time in proportion to a duration of the ON state of the switching element 404 as shown by the waveform (b), and thereby energy is charged into the primary coil 402.
During a period of B to C, when input of the pulse wave from the PWM control circuit 407 to the switching element 404 is ceased, as shown by the waveform (c), to turn the switching element 404 into an OFF state, the energy charged in the primary coil 402 begins to be charged into the capacitor 405, and amount of the current in the point xe2x80x9cOxe2x80x9d decreases with time, as shown by the waveform (b). The voltage of the primary coil 402 reaches its peak value as shown by the waveform (a), upon completion of the charge.
During a period of C to D, after completion of the charge into the capacitor 405, the energy charged in the capacitor 405 begins to be recharged into the primary coil 402 again, and amount of the current in the point xe2x80x9cOxe2x80x9d decreases with time, as shown by the waveform (b). The voltage of the primary coil 402 becomes zero as shown by the waveform (a), when the charge is completed.
During a period of D to E, when the charge to the primary coil 402 is completed, the energy charged in the primary coil 402 is about to start being recharged into the capacitor 405 again, and this recharge of the capacitor 405 is to begin through the ground side due to an effect of a positive-negative relation in polarity of the voltage across the primary coil 402. However, the capacitor 405 is not charged, but a current flows through the diode 406 having a low impedance, since the diode 406 is placed between the other side terminal of the primary coil 402 and the ground with the anode connected to the ground side. Therefore, although amount of the current flowing in the point xe2x80x9cOxe2x80x9d increases with time as shown by the waveform (b), the voltage of the primary coil 402 remains zero as shown by the waveform (a), since no energy is charged into the capacitor 405.
During a period of E to F, since the energy charged in the primary coil 402 has been discharged by the flow of current through the diode 406, amount of the current shown by the waveform (b) in the point xe2x80x9cOxe2x80x9d shall now remain theoretically zero, unless the switching element 404 is turned into an ON state with the waveform (c). In reality, however, amount of the current through the point xe2x80x9cOxe2x80x9d increases for a certain period of time as shown by the waveform (b).
A certain amount of energy is therefore charged in the primary coil 402 due to the increase of current through the point xe2x80x9cOxe2x80x9d.
Subsequently, during a period of F to G, the energy charged into the primary coil 402 begins to be charged to the capacitor 405 in the same manner as above, after the charge to the primary coil 402 is completed. Thus, amount of the current in the point xe2x80x9cOxe2x80x9d decreases with time, as shown by the waveform (b), and voltage of the primary coil 402 reaches its peak value as shown by the waveform (a), when the charge is completed.
During a period of G to H, after completion of the charge into the capacitor 405, the energy charged in the capacitor 405 begins to be recharged into the primary coil 402 again, and amount of the current in the point xe2x80x9cOxe2x80x9d decreases with time, as shown by the waveform (b). The voltage of the primary coil 402 becomes zero as shown by the waveform (a), when the charge is completed.
During a period of H to I, when the charge into the primary coil 402 is completed, the energy charged in the primary coil 402 is about to start being recharged into the capacitor 405 again. While amount of the current flowing in the point xe2x80x9cOxe2x80x9d increases with time as shown by the waveform (b), the voltage of the primary coil 402 remains zero as shown by the waveform (a), since no energy is charged into the capacitor 405.
During a period of I to J, amount of the current in the point xe2x80x9cOxe2x80x9d again increases for a certain period of time in the same manner as described above, as shown by the (b), and energy is hence charged in the primary coil 402.
During a period of J to K (A), after the charge to the primary coil 402 is completed, the energy charged in the primary coil 402 begins to be charged into the capacitor 405. Amount of the current in the point xe2x80x9cOxe2x80x9d decreases with time, as shown by the waveform (b), and the voltage of the primary coil 402 reaches its peak value as shown by the waveform (a), when the charge is completed. Since the switching element 404 is turned into an ON state during this period, as shown by the waveform (c), this becomes a new starting point and the same steps as above are repeated over again.
In the above composition, an output voltage of the secondary coil 408 changes depending on value of the voltage of the primary coil 402. The voltage of the primary coil 402 varies depending on a duration of time in the ON state of the switching element 404, and the longer the ON state, the greater the voltage.
As the pulse wave of the predetermined duration is input to the switching element 404 from the PWM control circuit 407 during this step, the voltage of the primary coil 402 becomes zero instantly at a timing the pulse wave goes on. Due to this sudden change in value of the voltage, amount of the current in the point xe2x80x9cOxe2x80x9d increases while producing undulation (W) shown by the waveform (b) in FIG. 17 during the ON state of the switching element 404. It has a substantial effect, especially if the voltage of the primary coil 402 changes to zero from a value greater than a voltage of the driving power supply in the timing the pulse wave turns on.
As has been described, there has been a problem with the above composition in that noises are generated on an display screen due to an influence of the undulation (W) in the current.
An object of the present invention is to provide a power supply circuit that suppresses undulation in current, and prevents screen noises from being generated on a display, for instance, when it is used as a load.
In order to achieve this object, the present invention provides a composition comprising;
a driving power supply connected to one side of terminals of a primary coil of transformer,
a first switching element,
a capacitor, and
a first diode, all connected to the other side of the terminals of the primary coil.
The first switching element is comprised of a first MOS type field-effect transistor (MOS FET) having a drain connected to the other side terminal of the primary coil, a source connected to a ground side, and a gate connected to a control circuit.
The capacitor has its one end connected to the other side terminal of the primary coil, and the other end connected to the ground side.
The first diode has its cathode connected to the other side terminal of the primary coil, and an anode connected to the ground side.
In addition, there is provided a noise suppression means between the transformer and the control circuit to restrict generation of noises.
With the composition as described above, generation of noises can be restricted.